"All songs that have double bass also have an "Expert+" difficulty.
In order to play expert+, rename the "notesexpert+.mid" to be your new mid file, double bass then be on the "Expert" difficulty(Drums)."

Credits:

Mustis666 (Guitars, less Century Child)
Tadracing99 (Drums and Century Child Guitars)
Raynebc (Nemo drums and guitars)